1. Field of the Invention
The invention relates to the field of digital TV (comprising cable TV, satellite TV, Terrestrial TV, and IPTV) and mobile multimedia, and more particularly to a digital TV conditional access system and a method of using the same for transmitting and receiving encrypted digital data.
2. Description of the Related Art
A conditional access (CA) system is used to control a subscriber's receipt of digital data, i.e., the subscriber can only receive the entitled digital television programs, including audio, video, and data, so that the operators can obtain profits from the subscribers via entitlement control and entitlement management.
In digital TV systems, the operators use a CA system to encrypt and transmit TV programs, and only the entitled subscribers can receive the transmissions. Aside from paid TV programs, the CA system can provide other value-added services comprising video on demand, information services, and internet. Thus, the CA system increases income for operators and becomes a basic and most important part of a digital TV system.
However, with the rapid development of computer and digital technologies, conventional CA systems have the following disadvantages:
1. Low Security
Conventional CA systems of digital TV are mainly based on Europe's DVB standard, which has been in place more than 10 years. Its working principle is described below. A front-end encrypted digital TV signal has a pair of periodically changeable secret keys, which was called control words (CW). The CA system encrypts and transmits the CW to the decryption apparatus of a receiver of digital TV content for decryption. Thus, the CW is obtained and transmitted to a descrambler which audio and video data streams are obtained and played by a play module.
At the time the standard was developed, considerations focused on how to transmit the CW safely to a receiver, but the decryption and diffusion of CW over shared by network technology was not expected. Therefore, the current situation, which includes highly developed network technology, poses, serious system loopholes that may be utilized by pirates, resulting in huge losses to digital TV operators.
The receiver of conventional CA system is generally a smart card. The decryption algorithm is saved in the smart card. Encrypted data are transmitted to the smart card from which the CW is obtained. The CW is transmitted to a descrambler via a receiver CA module. The CA module, as a separate component, is embedded in the software framework of a set-top box. The technology has security risks because CW can be intercepted at many points as shown in FIG. 1.
The first leak site is a communication point between the smart card and the receiver.
The second leak site is between the CA module and the descrambler.
The third leak site is the communication between the CA module and the memory (RAM).
No CA system with a CA module can avoid the first and second leak sites. To obtain the CW from the third leak site is very difficult and requires high cost. Thus, the first and second leak sites bring more security problems than the third leak site. The CW can be obtained from the first leak site by a simple device and can be obtained from the second leak site by separating the CA module from the set-top box. Either method is easily achieved existing computer technology and results in a leak in the CA. The relevant corresponding code may be readily found on the internet.
Every CA company has two secrets of the utmost importance: one is the CA algorithm and the other is the CA module. With the exception of network sharing, unless the CA algorithm is disclosed, the CA system is safe because the decryption of the CA algorithm is very difficult. However, the decryption of the CA module is very easy, resulting in the collapse of CA systems that use smart cards.
Based on the previous decryption of CA systems that use smart cards, the service life of the CA module is much longer than that of the CA algorithm. Often, the CA algorithm may be re-encrypted many times, but the CA module is unchanging. Thus, in the current situation of the ubiquity of highly developed network technology, it is more beneficial for pirates to encrypt the CA module.
For conventional CA systems that use smart cards, communication between the smart card and the set-top box is basically fixed. Any changes (upgrades) are minimal. For CA systems that have issued a large number of cards, changes are much more difficult to enact. Thus, even if the CA supplier has identified the leak site for the CW, the CA supplier almost has no methods for upgrading the CA security, which has a tremendous effect on the competitiveness of CA operators. Conventional CA systems that use a smart card include a set-top box in which are embedded many corresponding CA modules. The chip of the set-top box may vary, and the CA modules are embedded mainly by the set-top box manufacturer. Thus, the CA module data easily diffuse and may be leaked. Even if no leakage occurs, an ordinary hacker can successfully analyze the CA module from the program memory of the set-top box. Once analyzed, the data can be used for a long time. Thus, conventional CA systems that with smart cards have serious security risks.
2. Poor Universality
To improve the security index, conventional CA systems have many privacy mechanisms in place. Each CA supplier encrypts its own data as securely as possible using its own proprietary algorithms. Therefore, the set-top box manufacturers cannot integrate the many CA modules into a single set-top box, which makes the set-top box non-universal.
Furthermore, it is very difficult for CA suppliers to provide a special CA system for each CA operator. The CA algorithms provided by a single CA supplier are identical across operators. If a CA system used by an operator is decrypted, all other operators will be affected.
3. High Cost
In conventional CA receivers, a decryption control unit comprises at least a smart card interface circuit, a smart card reader device, and a special smart card, which greatly increase the receiving costs for digital TV, waste resources, and limit the popularization of digital TV.